Thanks to diplomatic immunity, foreign ambassadors and their staff occupy a unique place in society.

In short, they can do as they please regardless of the laws of their host country. They are immune from prosecution.

The principle, which the former Chilean dictator General Pinochet tried to invoke while under arrest in the UK, is age old. But it was only enshrined in international law with the 1961 Vienna Convention on Diplomatic Relations.

The convention, which is respected by almost every country in the world, derives from customs and procedures that have governed international relations over the centuries.

Free parking

"The rules are designed to ensure that diplomats - ambassadors and their diplomatic assistants - can perform duties without harassment," says Paul Wilkinson, Professor of International Relations at St Andrews University.

Powerful sentiments indeed, although in reality diplomatic immunity is most commonly viewed as the best "free parking" coupon in town.

General Augusto Pinochet : Seeking immunity

In July, the Foreign Office reported parking offences incurred by diplomatic missions rose by almost 50% - totalling 2,480 unpaid fines. Top of the list were Turkish envoys who clocked up 97 parking tickets in 1997.

But while diplomats can park as they please, Prof Wilkinson stresses immunity does not mean impunity at any cost.

Article 38 of the Vienna convention, states that some diplomatic agents shall enjoy immunity only "in respect of official acts performed in the exercise of his functions".

Article 41 says "persons enjoying such privileges. . . have a duty not to interfere in the internal affairs of [the receiving] state."

And while high-ranking diplomats remain out of reach of the long-arm of the law, a host country can always fall back on the right of expulsion.

Homeward bound

This was common practice during the Cold War when envoys suspected of spying were deemed persona non grata and given their marching orders home.

In the case of General Pinochet, Prof Wilkinson says it is "highly doubtful" that diplomatic immunity applies.

The convention makes no allowances for former heads of state, and besides, General Pinochet was visiting the UK for personal reasons. He is highly unlikely to have received one-off assurance of immunity from the British government prior to his visit.

UN convention

He is also unlikely to qualify under a United Nations convention on international protection. "This applies to existing heads of states, but not former ones."

And finally, says Prof Wilkinson, the diplomatic passport is just a misnomer.

"The diplomatic passport is more of a status symbol than anything. It does not automatically qualify him for blanket protection."

A hearing into General Pinochet's case is expected at Bow Street Magistrate's Court in London when he is well enough to leave the clinic where he was arrested.

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